Single triac reversible motor control

ABSTRACT

A single phase AC motor, biased to rotate in one direction, is made to stop when a controlled circuit tending to cause rotation in the opposite direction is energized by a half wave rectified alternating current and is made to rotate in the opposite direction when the controlled circuit is energized by alternating current. A triac rectifies the control current when fired in only one quadrant and passes alternating current when fired in two quadrants, a modulated DC gating current determining whether the triac is fired in one quadrant, in two quadrants or in none. Speed control may be exercised by controlling the phase angle of the firing.

United States Patent Horton June 27, 1972 SINGLE TRIAC REVERSIBLE MOTORCONTROL inventor:

Assignee:

Filed:

Appl. No.:

us. or ..318/208, 318/345 rm. Cl. ..H02p 7/62 Field of Search ..3 l8/208, 222, 223, 284, 288,

References Cited UNITED STATES PATENTS 10/1968 Didwall.... ..3l8/2088/1969 Kompelien ..t ..3l8/345X 3,588,647 6/1971 Harwell ..3l8/345 XPrimary Examiner-Harold Broome Assistant Examiner-W. E. Duncanson, Jr.Attorney-A. Richard Koch ABSTRACT A single phase AC motor, biased torotate in one direction, is made to stop when a controlled circuittending to cause rotation in the opposite direction is energized by ahalf wave rectified alternating current and is made to rotate in theopposite direction when the controlled circuit is energized byaltemating current. A triac rectifies the control current when fired inonly one quadrant and passes alternating current when fired in twoquadrants, a modulated DC gating current determining whether the triacis fired in one quadrant, in two quadrants or in none. Speed control maybe exercised by controlling the phase angle of the firing.

10 Claims, 3 Drawing Figures P'A'T'ENTElJJum I972 SHEET 10F 2 INVENTORALFRED A. Homo/v AGENT SINGLE TRIAC REVERSIBLE MOTOR CONTROL BACKGROUNDOF THE INVENTION This invention is concerned with motor control systemsfor positioning a load in accordance with a condition sensitive plishedthrough control of an electric motor in accordance with the sense and/orthe amplitude of an error signal. When there is no error the motorstops. This invention is concerned with control of single phase ACmotors in such service.

Shaw, in US. Pat. No. 2,334,447 disclosed that a reversing shaded polemotor may be biased by shading rings on one set of shading poles torotate in one direction, may be rotated in the opposite direction byshorting shading windings on the other set of shading poles, and may bestopped by closing the shading windings through a resistance such thatthe effects of the shading poles are balanced.

Triacs and other bidirectional thyristors are used to control the supplyof alternating current to a load. For most applications it is considereddesirable that they conduct current symmetrically in either direction.It is, however, well known that less gate current is required to fire atriac when the load current and gate current are flowing in thesamedirection than when flowing in opposite directions and that less gatecurrent is required when the gate is at positive potential with respectto the common terminal. These characteristics are generally consideredto be a nuisance and great efforts have been made to eliminate or reducethem. A more complete description of a bidirectional controlled switchby Gentry, Scace and Flowers, entitled Bidirectional Triode P-N-P-NSwitches," may be found in Proceedings of the IEEE, Vol. 53, No. 4,pages 355-369, April 1965.

SUMMARY OF THE INVENTION This invention takes advantage of theassymmetrical firing characteristics of a bidirectional thyristor tocontrol by a single such thyristor the starting, stopping and directionof motion of a single phase alternating current motor. Speed is alsocontrolled as a result of the same assymmetrical characteristics.Control over the thyristor is exercised by a condition sensitivecircuit. The control is simple and inexpensive.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagramfor a preferred embodiment of this invention.

FIG. 2 comprises graphs showing the volt-ampere characteristic of triacsor similar bidirectional thyristors at various gate currents.

FIG. 3 comprises graphs showing the voltages and currents plottedagainst time at various gate currents.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As shown in FIG. I, areversible shaded pole motor 1 has a rotor 2, a primary winding 3energized from an alternating current power supply, a shorted shadingwinding or ring 4 providing biasing torque tending to turn the rotor 2in what will be called reverse direction, and a controlled shadingwinding 5 selectively shorted to provide torque tending to turn therotor 2 in forward direction in opposition to the biasing torqueprovided by the shading ring 4. A bidirectional thyristor 7, an examplebeing a triac, which has a pair of power terminals, identified as commonterminal 8 and load terminal 9, and a gate terminal 10. The shadingwinding 5 is connected between terminals 8 and 9 to form a controllablecircuit.

A regulated direct current power supply is connected between a hot line12 (shown as and a reference line 13 (shown as An unbalanced bridge 15has one arm comprising a fixed resistor 16 connected between the hotline 12 and a junction 17, a second arm comprising an adjustable setpoint resistor 18, a variable feedback resistor 19 and a conditionsensitive variable resistor 20 connected between the junction 17 and thereference line 13, and an adjustable calibrating potentiometer 21connected between line 12 and 13 to provide the other two arms of thebridge. A wiper 22 at any predetermined position on the calibratingpotentiometer 21 supplies a fixed reference voltage V to a differentialinput operational amplifier 24 at the non-inverting terminal thereof,while junction 17 supplies a condition sensitive variable voltage V tothe inverting input terminal 26 to provide between the input terminalsan error signal voltage V,;. The output terminal 27 is connected to thenon-inverting input terminal 25 through a feedback resistor 28 toestablish the gain A of the amplifier 24. The output terminal 27 alsosupplies its output voltage V to a control terminal 30, V being equal toAV A voltage divider 31, comprising a coupling resistor 32 and a gateshunting resistor 33 in series between control terminal 30 and thereference line 13, serves as a substantially constant load for theamplifier and provides at the junction 34 between resistors 32 and 33 agate voltage V which is a substantially constant fraction of the outputvoltage V Junction 34 is connected to gate terminal 10 and referenceline 13 is connected to the common terminal 8. The gate shuntingresistor 33 is chosen to be of sufficiently high resistance to preventaccidental dv/dt firing of the thyristor 7, but of sufficiently lowresistance so that the gate current flowing through the gate and commonterminals 10 and 8 of thyristor 7 will have little effect on gatevoltage V When an amplified error signal in the form of output voltage Vappears at control terminal 30, a gate current I flows from junction 34through the thyristor 7 between gate terminal 10 and common terminal 8to reference line 13. If such gate current is of sufficient amplitude tofire the thyristor, the controlled circuit from the shading winding 5through the thyristor between load terminal 9 and common terminal'8 backto shading winding 5 will be closed. When the shading winding 5 is thusshorted by thyristor 7, an alternating voltage V induced in the shadingwinding by the alternating current in primary winding 3 will cause analternating load current I to flow through the controlled circuit andenergize the winding 5 to produce torque tending to produce forwardrotation of rotor 2. How this torque is controlled is explained later.

The circuit described above is an open loop controller. If a closed loopcontroller is desired, a feedback may be provided as by connection 36from the rotor 2, or the motor driven load 37, to a wiper 38 on thefeedback resistor 19.

It is well known that the voltage V at which a thyristor will break overis dependent upon the amplitude of the gate current 1 between the gateand common terminals. It is also known that with a given gate current abidirectional thyristor will break over at a lower voltage when the gateand load terminals are of the same polarity than when of oppositepolarity. For bidirectional thyristors it is likewise known that thebreakover will occur at lower voltage when the gate is positive withrespect to the common terminal. In order to over come thesecharacteristics thyristors are generally phase fired by pulses ofsuflicient amplitude that the thyristor can be fired at substantiallyany time during the cycle by delaying the imposition of the gating pulsefor the desired time after the controlled alternating load voltage haspassed through zero. The thyristor is thus fired at substantially thesame phase angle regardless of polarity of the load voltage. When onlyon-off operation is desired a direct current gating circuit providing agate current of sufiicient amplitude to fire the thyristor early in thecycle regardless of load voltage polarity may be employed. Neither ofthese methods could produce the results obtained by this invention.

As shown in FIG. 2, when the gate current 1 is zero, which condition ishereinafter designated as 1 the load current I will be substantiallyzero, which condition is hereinafter age V and the zero load current 1is shown in FIG. 3 along with the root-mean-square load current, oreffective load current 1 which is likewise substantially zero. As thegate current 1 is increased in amplitude to I the breakover voltage Vbetween terminals 9 and 8 is reduced to the positive maximum controlledvoltage +V at which value the breakover voltage is herein designated asV The triac 7 is then fired to conduct load current 1 from load terminal9 to common terminal 8 for the remaining quarter cycle of positiveinduced voltage V. The effective load current I 1 under this conditionis one-quarter of the maximum effective load current l 6 as laterdefined. As the gate current is further increased to the breakovervoltage V is less than the maximum induced voltage V and the triac 7will be fired earlier in the half cycle when the induced voltage Vreaches V The resultant load current I has a higher effective value I 2because I, flows for a greater portion of the cycle than 1 When the gatecurrent is increased to l at which current the breakover voltage V issubstantially zero, oad current 1;, will flow for substantially theentire half cycle and I 3 will be half of I Whenever the gate currentexceeds 1 the load current will flow for substantially the entire halfcycle during which the load terminal 9 is positive with respect to thecommon terminal 8. At a gate current 1., the thyristor 7 will be firedfor conduction from common terminal 8 to load terminal 9 with thebreakover voltage V equal to V so that load current 1 will flow duringthe last half of the negative half cycle as well as during the entirepositive half cycle, and the efi'ective voltage I 4 will beapproximately 75 percent of I At higher gate currents such as I thebreakover voltage V will be less than V so that load current I: willbegin to flow earlier in the negative half cycle, again resulting in ahigher effective load current I Whenever the gate current is increasedto or beyond I at which current the breakover voltage V is substantiallyzero, load current 1 will flow for substantially the entire cycle andthe effective current l which is the maximum effective current, will besubstantially 0.707

When the shading winding 5 is shorted during substantially the entirecycle, due to a gating current I or more firing the thyristor 7 atsubstantially zero angle on both half cycles, the maximum effective loadcurrent 1 6 will flow through the winding, producing the maximum torquetending to move the rotor 2 in forward direction. This maximum torquewill be hereafter referred to as full torque. When the gating current isreduced to I 3 is only half of l B and the torque produced isproportionately reduced to half of full torque, commonly referred to ashalf torque. The shading ring 4 is selected such that the voltageinduced in it by the alternating current in the primary winding 3 willcause an alternating current in the ring of an amplitude to produce halftorque tending to move the rotor 2 in reverse direction. When thisreverse half torque is balanced by a forward half torque produced as aresult of a gate current between 1 and 1 firing the thyristor 7 for half.cycles of only one polarity, there will be no tendency to move therotor in either direction. If the gate current is less than l so thatthe thyristor 7 is not fired at any time, no current will flow throughthe shading winding 5 and no forward torque will be produced to opposethe reverse torque and the rotor 2 will tend to be moved in reversedirection at normal speed. If the gate current is equal to or exceeds lso that substantially full alternating current flows through shadingwinding 5 and forward full torque is produced in opposition to thereverse half torque, a resultant forward half torque will tend to movethe rotor in forward direction at normal speed.

By refining the above motor reversing control circuit, some measure ofspeed control is also possible. The forward torque produced by loadcurrent I through the shading winding 5 is proportional to the effectivevalue of the load current, or I When the gate current is between I and Ia corresponding load current between 1 and 1, will flow forbetweenonequarter and one-half cycle, producing an effective current between I1 and I which will provide between onequarter and one-half torque. Suchtorque will not balance or overcome the reverse half torque but willcause a resultant reverse torque between zero and one-quarter torque,which will only provide a reduced speed in reverse direction. In likemanner, when the gate current is between and I the forward torque willovercome the reverse torque, producing a resultant forward torquebetween one-quarter and one-half torque, which will provide only areduced speed in forward direction.

By eliminating the shading ring 4, a unidirectional shaded pole motor isobtained and with no biasing torque to overcome a range of torques fromzero to full torque is available to move the rotor 2 in a predetermineddirection. Under this condition half torque will provide movement in thepredetermined direction but at a lesser speed, dependent upon the drivenload 37, than when full torque is available.

This invention may also be employed to control linear motors in which areciprocable member replaces the rotatable member, or rotor, movinglinearly with respect to the windings in response to electromagneticforces produced by current flow in the windings.

The embodiments described are for purposes of illustration only and notto define the scope of this invention which is limited only by theclaims.

I claim:

1. A motor control system comprising a reversible electric motor, analternating current power supply connected to energize the motor, amovable member in said motor, means providing a biasing force tending tomove the member in one direction, a controllable circuit when closedproducing a force tending to move the member in opposite to said onedirection, a bidirectional thyristor having load, gate and common terminals, said thyristor connected at the load and common terminals insaid controllable circuit to selectively close the circuit, means forproducing a gating signal of variable amplitude between said gate andcommon terminals, a first amplitude of said gating signal firing thethyristor into conduction only when said gate and load terminals are oflike polarity with respect to the common terminal whereby saidcontrollable circuit being energized by half wave rectified alternatingcurrent produces sufficient force to substantially balance the biasingforce and efiectively eliminate any tendency to move the member, and agreater second amplitude of said gating signal firing the thyristor intosubstantially continuous conduction whereby said controllable circuitbeing energized by alternating current produces force overcoming thebiasing force and supplying a resultant force tending to move the memberis said opposite direction.

2. A motor control system according to claim 1 wherein said gatingsignal having an amplitude less than said first amplitude phase firesthe thyristor to control the speed of motion of said member in said onedirection.

3. A motor control system according to claim 1 wherein an amplitude ofgreater than said first amplitude phase fires the thyristor to controlthe speed of motion of said member in said opposite direction.

4. A motor control system according to claim 1 wherein said biasingmeans comprises a continuously energized winding providing a magneticforce tending to move the member in said one direction.

5. A motor control system according to claim 1 additionally comprising agate shunting resistor connected between the gate and common terminals.

6. A motor control system comprising a reversible shaded pole motorhaving a rotor, a primary winding and first and second shading windings,an alternating current power supply connected to energize said primarywinding, said first shading winding continuously shorted to produce afixed biasing torque tending to rotate the rotor in one direction, saidsecond shading winding controllable to produce a variable torque tendingto rotate the rotor in opposite to said one direction, a bidirectionalthyristor having load, gate and common terminals, said thyristorconnected at the load and common terminals in series with said secondshading winding to selectively short said second shading winding, andmeans for producing a gating signal of variable amplitude between saidgate and common terminals, a first amplitude of said gating signalfiring the thyristor into conduction only when said gate and loadterminals are of like polarity with respect to the common terminal toenergize said second shading winding with half wave rectifiedalternating current producing sufficient torque to substantially balancethe biasing torque and efiectively eliminate any tendency to rotate therotor, and a greater second amplitude of said gating signal firing thethyristor into substantially continuous conduction to energize saidsecond shading winding with alternating current producing torque toovercome the biasing torque and supply a resultant torque tending torotate the rotor in said opposite direction.

7. A motor control system comprising an electric motor, an alternatingcurrent power supply connected to energize said motor, a movable memberin the motor, a controllable circuit producing a variable force tendingto move said member in a predetermined direction, a bidirectionalthyristor having load, gate and common terminals, means for producing agating signal of variable amplitude between said gate and commonterminals, means for connecting said thyristor at the load and commonterminals in the controllable circuit to selectively close said circuit,a first amplitude of the gating signal firing said thyristor intoconduction only when said gate and load terminals are of like polaritywith respect to the common terminal to energize said controlled circuitwith half wave -said second force tending to move said member in thepredetermined direction.

9. A motor control circuit for a motor controlled by the opening andclosing of a controllable circuit energized from an alternating currentpower supply, said motor control circuit comprising a bidirectionalthyristor having load, gate and common terminals, means for producing adirect current gating signal of variable amplitude in response to avariable condition, said gate and common terminals receiving the gatingsignal, and means for connecting said load and common terminals into thecontrollable circuit whereby conduction and non-conduction of saidthyristor closes and opens the controllable circuit, a first amplitudeof said gating signal firing the thyristor into conduction only whensaid gate and load terminals are of like polarity with respect to thecommon terminal, and a greater second amplitude of said gating signalfiring the thyristor into substantially continuous conduction.

10. A motor control circuit according to claim 9 wherein an amplitudeless than said second amplitude of the gating signal phase fires thethyristor.

1. A motor control system comprising a reversible electric motor, analternating current power supply connected to energize the motor, amovable member in said motor, means providing a biasing force tending tomove the member in one direction, a controllable circuit when closedproducing a force tending to move the member in opposite to said onedirection, a bidirectional thyristor having load, gate and commonterminals, said thyristor connected at the load and common terminals insaid controllable circuit to selectively close the circuit, means forproducing a gating signal of variable amplitude between said gate andcommon terminals, a first amplitude of said gating signal firing thethyristor into conduction only when said gate and load terminals are oflike polarity with respect to the common terminal whereby saidcontrollable circuit being energized by half wave rectified alternatingcurrent produces sufficient force to substantially balance the biasingforce and effectively eliminate any tendency to move the member, and agreater second amplitude of said gating signal firing the thyristor intosubstantially continuous conduction whereby said controllable circuitbeing energized by alternating current produces force overcoming thebiasing force and supplying a resultant force tending to move the memberis said opposite direction.
 2. A motor control system according to claim1 wherein said gating signal having an amplitude less than said firstamplitude phase fires the thyristor to control the speed of motion ofsaid member in said one direction.
 3. A motor control system accordingto claim 1 wherein an amplitude of greater than said first amplitudephase fires the thyristor to control the speed of motion of said memberin said opposite direction.
 4. A motor control system according to claim1 wherein said biasing means comprises a continuously energized windingproviding a magnetic force tending to move the member in said onedirection.
 5. A motor control system according to claim 1 additionallycomprising a gate shunting resistor connected between the gate andcommon terminals.
 6. A motor control system comprising a reversibleshaded pole motor having a rotor, a primary winding and first and secondshading windings, an alternating current power supply connected toenergize said primary winding, said first shading winding continuouslyshorted to produce a fixed biasing torque tending to rotate the rotor inone direction, said second shading winding controllable to produce avariable torque tending to rotate the rotor in opposite to said onedirection, a bidirectional thyristor having load, gate and commonterminals, said thyristor connected at the load and common terminals inseries with said second shading winding to selectively short said secondshading winding, and means for producing a gating signal of variableamplitude between said gate and common terminals, a first amplitude ofsaid gating signal firing the thyristor into conduction only when saidgate and load terminals are of like polarity with respect to the commonterminal to energize said second shading winding with half waverectified alternating current producing sufficient torque tosubstantially balance the biasing torque and effectively eliminate anytendency to rotate the rotor, and a greater second amplitude of saidgating signal firing the thyristor into substantially continuousconduction to energize said second shading winding with alternatingcurrent producing torque to overcome the biasing torque and supply aresultant torque tending to rotate the rotor in said opposite direction.7. A motor control system comprising an electric motor, an alternatingcurrent power supply connected to energize said motor, a movable memberin the motor, a controllable circuit producing a variable force tendingto mOve said member in a predetermined direction, a bidirectionalthyristor having load, gate and common terminals, means for producing agating signal of variable amplitude between said gate and commonterminals, means for connecting said thyristor at the load and commonterminals in the controllable circuit to selectively close said circuit,a first amplitude of the gating signal firing said thyristor intoconduction only when said gate and load terminals are of like polaritywith respect to the common terminal to energize said controlled circuitwith half wave rectified alternating current to produce a first forcetending to move said member in the predetermined direction, and agreater second amplitude of the gating signal firing said thyristor intosubstantially continuous conduction to energize said controlled circuitwith alternating current to produce a second force tending to move saidmember in the predetermined direction, said second force beingsubstantially double the first force.
 8. A motor control systemaccording to claim 7 wherein the gating signal having an amplitude lessthan said second amplitude phase fires the thyristor to produce a forceless than said second force tending to move said member in thepredetermined direction.
 9. A motor control circuit for a motorcontrolled by the opening and closing of a controllable circuitenergized from an alternating current power supply, said motor controlcircuit comprising a bidirectional thyristor having load, gate andcommon terminals, means for producing a direct current gating signal ofvariable amplitude in response to a variable condition, said gate andcommon terminals receiving the gating signal, and means for connectingsaid load and common terminals into the controllable circuit wherebyconduction and non-conduction of said thyristor closes and opens thecontrollable circuit, a first amplitude of said gating signal firing thethyristor into conduction only when said gate and load terminals are oflike polarity with respect to the common terminal, and a greater secondamplitude of said gating signal firing the thyristor into substantiallycontinuous conduction.
 10. A motor control circuit according to claim 9wherein an amplitude less than said second amplitude of the gatingsignal phase fires the thyristor.